The activity that occurs when two tectonic plates interact with each other can have a major impact on the landscape of the Earth. Although the process can take millions of years, the landforms that are formed in this way are some of the most impressive.
On a compressional plate margin, two plates move together and can create fold mountains. This may involve two continental plates moving together or a continental plate and an oceanic plate moving together, forcing sedimentary rocks upwards into a series of folds. Fold mountains are usually found along the edges of continents as it is there that the greatest sedimentary rock deposits accumulate. When tectonic plates collide, layers of accumulated rock crumple and fold. Fold mountains that are between 10 and 20 million years old, such as the Himalayas, are known as young fold mountains and those that are more than 200 million years old, including the Appalachians and the Urals, are called old fold mountains.
Ocean trenches are formed in one of two ways: either through two tectonic plates colliding or through continental land masses colliding with the moving sea floor. This can create a trench on the sea floor side and a mountain range on the side of the land mass, as can be seen with the Andes. These trenches are long, narrow valleys in the deepest areas of the ocean. The deepest ocean trench is the Marinas Trench, reaching a depth of almost 36,000 feet below sea level.
The subduction process that occurs when an oceanic plate converges with another oceanic plate can lead to volcanoes being formed. The volcanic debris and lava build up on the ocean floor over millions of years and eventually results in a submarine volcano rising above sea level to create an island. A curved chain of these volcanoes usually occurs in these cases, known as island arcs. The magma that forms these arcs is a result of the descending plate or the overlying oceanic lithosphere melting partially.
On divergent boundaries, plates move away from each other, creating a new crust as magma is pushed up from the mantle. The movement of the tectonic plates transports the newly formed crust away from the crest of the ridge in both directions. The underwater mountain range known as the Mid-Atlantic Ridge is an example of this. The Mid-Atlantic Ridge spreads at an average rate of 2.5 centimeters each year, having resulted in thousands of kilometers of plate movement and creating the mountains that exist today over the course of millions of years.